Exercise is like medicine for the heart, and just like with medication, you need the right "dose" for it to be effective.

Men needed to do roughly nine hours of exercise to see a 30% reduction in their heart disease risk – while women only needed to do around four hours.

In addition to helping you to know whether you’re meeting general recommendations for exercise, knowing the intensity of a given workout session can be of benefit in other ways. First, the body uses ...